1. Field of the Invention
The present invention relates to position sensors which have a scale with a mechanical period and which emit sensor signals with a period which depends on the mechanical period such as e.g. a linear variable differential transformer (LVDT) or a rotational variable differential transformer (RVDT) and in particular to the preparation of such a sensor signal of a position sensor for output to an appropriate evaluation unit.
2. Description of the Related Art
Examples of position sensors which employ a mechanical scale to perform a path measurement or an angular measurement are linear variable differential transformers and rotational variable differential transformers, described hereafter as resolvers, and special arrangements of magnetoresistive resistors or Hall sensors which are used to measure a path or an angle of rotation α for mechanical arrangements or machines. These sensors supply two output signals which vary depending on the mechanical position, so that the position referred to a period section of the mechanical scale can be unambiguously determined from the signals.
FIG. 1a and FIG. 1c show examples of two different arrangements for measuring the linear position, while FIG. 1b shows an arrangement for measuring an angle of rotation. FIG. 1a exhibits an excitation coil 10 and two measurement coils 20 and 30 and a measurement object 40 with suitable material properties, such as e.g. a suitable magnetic susceptibility, which is arranged between the excitation coil 10 on one side and the measurement coils 20 and 30 on the other and which is moveable linearly along an axis 50. The arrangement is so conceived that a linear displacement of the measurement object 40 or the excitation coil 10 causes a change in the coupling relationship between the excitation coil 10 and the measurement coil 20 and between the excitation coil 10 and the measurement coil 30. An excitation voltage on the excitation coil 10 therefore produces signals in the measurement coils 20 and 30 which are in quadrature to each other. The position of the measurement object 40 can be defined as an angle α which determines the relationship between the two measurement signals, as will be explained below.
The arrangement shown in FIG. 1b corresponds to the arrangement shown in FIG. 1a with the exception of the measurement object 40. In this case the measurement object is represented by a rotatable body 50. When the body 50 is rotated the relationship between the measurement signals registered in the measurement coils 20 and 30 varies as in the arrangement in FIG. 1a according to the angle of rotation α, whereby the angle of rotation α can be determined.
FIG. 1c shows an alternative arrangement to that in FIG. 1a. It has magnetoresistive sensors 60 and 70 and a magnetic scale 80 constitutes the linearly displaceable measurement object. The magnetic scale 80 has two suitably oriented magnetic regions which generate opposing magnetic fields at the location of the magnetoresistive sensors 60 and 70, these regions being represented in FIG. 1c by four bar magnets 80a, 80b, 80c and 80d, the orientation of which alternates from one to the next. When the scale 80 is displaced along an axis 90 the magnetic field at the location of the magnetoresistive sensors 60 and 70 changes and so also therefore does the electrical resistance in such a way that the signals measured at the sensors 60 and 70 are in quadrature to each other.
In consequence, the variation of the signals is characterized in the first instance by the fact that they are substantially in quadrature to each other. FIG. 2 shows the connection between the value α on the one hand and the measurement signals at the coil 20 and the coil 30 on the other in relation to an excitation voltage U0 for the measurement arrangement shown in FIG. 1b. The connection is also essentially true for the arrangement shown in FIG. 1c and FIG. 1a. 
As can be seen from FIG. 2, the periodic signals Usin and Ucos defined in terms of the mechanical period lPER of the mechanical scale can be described by the following equations:
                              U          ⁢                                          ⁢          sin                =                              U            0                    ⁢                                          ⁢                      sin            ⁡                          (                                                2                  ⁢                  πα                                                  1                  PER                                            )                                                          Eq        .                                  ⁢        1                                          U          ⁢                                          ⁢          cos                =                              U            0                    ⁢                                          ⁢                      cos            ⁡                          (                                                2                  ⁢                  πα                                                  1                  PER                                            )                                                          Eq        .                                  ⁢        2            where U0 may be a direct or alternating voltage or a direct or alternating current, such as U0=Upp cos(ωt), Upp being the amplitude of the alternating voltage U0.
FIG. 3 shows the signal profiles of the measurement signals Usin and Ucos of the sensors of FIGS. 1b and 1c as a function of the angle α or the linear displacement α. As can be seen, the variation of these signals is characterized by the fact that they are in quadrature to each other, i.e. they relate to one another like cosine and sine, and that the signals Ucos and Usin are periodic and that their period is equal to the mechanical period lPER of the mechanical scale. In the case of FIG. 1b the mechanical period lPER is e.g. equal to a full rotation, i.e. 360°, and in the case of FIG. 1c it is equal to the distance between two magnets with the same orientation. In other words, the signals Usin and Ucos only have a unique relationship to the measurement value α within a period lPER and they repeat themselves periodically when a number of period sections of length lPER succeed one another, e.g. in the case of two rotations.
In FIG. 4 the variation with time of the sensor signals Ucos and Usin is shown for the case of a constant rotational or translational movement. As can be seen, the signals Usin and Ucos are periodic signals which cover the value range of the signal profiles shown in FIG. 3 in successive periods. The time period length of the signals Ucos and Usin is equal to the quotient of the mechanical period lPER and the linear velocity or angular velocity v. After a time duration of lPER/v the relative rotation or displacement of the scale to the position sensor has covered a mechanical period lPER. A non-constant rotation or speed of motion also produces periodic signals which do not, however, have a constant period but a fluctuating period.
Since nearly all the controls and regulators of mechanical systems are to an increasing extent realized digitally, the output signals Usin and Ucos of the sensors must normally be digitalized. To find a digital equivalent αDIG of the position α, the ratio of Usin to Ucos must be evaluated. The required relationship is generally as follows:
                              α          DIG                =                  arctan          ⁡                      (                                          U                ⁢                                                                  ⁢                sin                                            U                ⁢                                                                  ⁢                cos                                      )                                              Eq        .                                  ⁢        3            
Some evaluation methods digitalize both voltages Usin and Ucos and then calculate the arctangent digitally, others digitalize the two voltages Usin and Ucos simultaneously and hereby form the digital value αDIG directly.
To transmit the sensor signals as generated by one of the sensors in FIG. 1a–1c and as shown as examples in FIG. 4 to an evaluation unit, where they are evaluated, e.g. digitalized, the solutions shown in FIGS. 5, 6 and 7 are traditionally used to connect the position sensor to an evaluation unit. In the following description of FIG. 5 to 7 it should be noted that identical elements in the drawings are denoted by the same reference numerals and that a repetition of the description of these identical elements is dispensed with.
FIG. 5 to 7 show in each case a position sensor 100, which, by means of a scale 110 with a mechanical period lPER, registers a relative linear displacement 120 of the scale 110 in relation to the position sensor 100 or a displacement of the position sensor 100 in relation to the scale 110. In the case of FIG. 5 the position sensor is connected directly to an evaluation unit 130, the position sensor 100 being connected to the evaluation unit 130 via four transmission lines 140a, 140b, 140c and 140d in order to transmit the sensor signal Usin and the sensor signal Ucos differentially to the evaluation unit 130.
To reduce the transmission errors arising during the transmission from the position sensor 100 to the evaluation unit 130 due to the length of the transmission lines 140a–140d, in the solution for transmitting the sensor signals which is shown in FIG. 6 analog line drivers 150a and 150b, which are connected to the sensor 100 over lines 155a, 155b, 155c and 155d and which guarantee a better transmission through amplification or preparation of the sensor signals Usin and Ucos, are inserted in front of the transmission lines 140a–140d. 
The solution for transmitting the sensor signals of the position sensor 100 to the evaluation unit 130 shown in FIG. 7 increases the reliability of the transmission of these signals by preparing or digitalizing them before transmitting them to the evaluation unit 130. Digitalization is achieved by means of an analog/digital converter 160 connected to the position sensor 100 and which receives the analog sensor signals Usin and Ucos, which are fed in differentially, digitalizes them, amplifies them in digitalized form by means of line drivers 160a and 160b at an output stage of the converter and sends them, in their digitalized and amplified form, to the evaluation unit 130 on transmission lines 170a and 170b. In contrast to the transmissions according to FIGS. 5 and 6, the sensor signals Usin and Ucos are already digitalized in the immediate vicinity of the position sensor 100 by the analog/digital converter 160 rather than later in the evaluation unit 130. The digital evaluation unit 130 can perform the previously mentioned calculation of the arc tangent on the basis of the digital sensor signals.
The evaluation of the sensor signals Usin and Ucos is substantially independent of the variation of the excitation voltage U0, which means that the interference which occurs equally on both signals or on both line pairs 140a, 140b or 140c and 140d or on both lines 170a and 170b, has almost no effect on the evaluation result. Interference which is superimposed on only one of the two sensor signals Usin and Ucos, on the other hand, directly affects the measurement result. To achieve the best possible result it is therefore desirable to use the smallest possible mechanical period lPER so that the quotient of the sensor signals Usin and Ucos has to be determined only very imprecisely and only a few points of a period have to be evaluated at a certain resolution, thus minimizing the effect of unsymmetrical interference on the evaluation. An extreme case is that where the zero transitions and the maxima of both signals are sampled. In the digital representation of the sensor signals with one bit each the result is the increment signal which is common in industrial control engineering.
However, some problems stand in the way of achieving the smallest possible mechanical period in order to counteract asymmetrical interference with the in-quadrature signals. In the first place, mechanical and manufacturing problems mean that it is not always possible to make the scales sufficiently small, to attach them or to read them. Secondly, the frequency of the sensor signals of the position sensor 100 becomes very high at high rotational or translational velocities. In FIG. 4, in which exemplary sensor signals Ucos and Usin are shown as a function of time t for the case of a constant rotational or translational motion, the sampling time Δt between two samplings in the evaluation unit is also shown. As far as the maximum translational or rotational velocity is concerned, the evaluability of the sensor signals Ucos and Usin is therefore restricted by the bandwidth and the sampling speed Δt of the evaluation electronics or the evaluation unit, so that e.g. Δt ≦½lPER/v must hold true. A lower bound for the mechanical period also arises from the fact that higher frequency signals in the region of several megaherz can no longer be transmitted with little loss in the transmission lines. In addition to the interference from outside there is now also distortion of the signals due to line losses.
Taking into account the bandwidth and the sampling speed of the evaluation electronics as well as the transmission losses of the sensor signals, for any particular application characterized by a certain desired resolution of αDIG, the maximum actual translational or rotational velocity, the length of the transmission path over which the sensor signals must be transmitted to the evaluation unit, and the amount of interference incident along the transmission path, there is for the position sensor a mechanical period which generates sensor signals which can be transmitted and evaluated optimally. This mechanical period will be described as the electrically optimal period in what follows.
On the other hand, however, there exists a mechanical period which would be optimal as regards manufacture, attachment and readability from the mechanical point of view, and which is usually greater than the electrically optimal period and which will be called the mechanically optimal period in what follows. At high translational velocities it is, however, equally possible that the sensor is moved very quickly and that the optimal mechanical period from the mechanical viewpoint is smaller than the electrically optimal period.
Deviations of the mechanically optimal period from the electrically optimal period occur primarily when a position sensor of an existing machine control system is replaced. If, for example, a sensor of greater accuracy or of higher resolution and which operates according to a different principle is installed in an existing machine control system, the period of the output signal changes as well and is no longer optimally adjusted.
DE 19815438A1 relates to a position measurement device and a method for operating a position measurement device. In particular the use of a signal period variation unit is described which is connected between a position measurement device and an evaluation unit in order to increase the signal frequencies of periodically modulated analog increment signals from the position device or to virtually decrease the geometric period. One embodiment of the signal period variation unit consists of two interpolation devices, which receive the analog increment signals output by the position device and which are out of phase by 90° and which produce digital words which indicate a position value, a conversion table which, via the digital words or the position value, accesses a number of conversion tables in which each position value among the digital words is assigned a particular modified position value so that the result is always a sine- or cosine-shaped signal profile with increased signal period, and two D/A converters, which generate quasi-analog sine- and cosine-shaped increment signals from the table entries which are read out and passes them on to the evaluation unit. In another embodiment, instead of there being two digital words indicating the instantaneous position value, an interpolation unit and a direction recognition unit generate a pulsed digital signal and a direction-indicating direction signal from the analog signals. These signals are fed into an address counter unit 24 which, depending on the desired signal period variation factor, advances by a predetermined number of entries in a conversion table 21A and a conversion table 21B using an address pointer 34A or 34B, the conversion tables storing in digital form signal amplitude values of a sine or cosine function. By increasing the step size when advancing within the tables 21A and 21B the simulated signal period can be adjusted step by step.
EP 0463561B1 and U.S. Pat. No. 5,347,355 describe a signal processing method and a signal processing device and also a system, such as e.g. a displacement detection device in which they are used. From the sine and cosine signals S1 and C1 of a displacement detection device and with the aid of adders, multipliers etc., i.e. using analog circuitry, sine and cosine signals are generated whose signal frequency is a whole number of times greater. The circuit for frequency doubling is incorporated in a processing circuit which, on its input side, doubles the frequency of the sine and cosine signals, which are 90° out of phase, and then converts them by zero transition analysis into division pulses whose number corresponds to the distance between an optical system 101, 102, 104–107 and a diffraction grating 103.
JP 02099826A describes a device for processing a signal of an encoder wherein detection signals with phases from 0 to 90 degrees emitted by detectors are first converted into pulse signals and are then converted into binary code signals in an incremental/decremental counter. By extracting binary code signals of optional weighting in data selection units, pulse signals a and b are obtained which correspond to different frequency divisions of the detection signals. In particular the pulse frequency of the signal a obtained by extracting the slower changing binary code signals is a factor 2 smaller than that of signal b obtained from the binary code signals changing with higher frequency, the lowest valued of these changing with the same frequency as the signal b. A pulse signal c which is phase-shifted by 90° relative to the pulse signal b and which has the same, i.e. halved, frequency as the signal a, is obtained by XORing of the signals a and b. The signals a and c thus generated ensure high precision even when a control circuit with low signal processing speed is employed.